1. Field of the Invention
The present invention relates to communication systems with multiple stand-alone stations that transmit data using multiple frequency coding.
2. Description of Related Art
Communication networks provide a system by which a number of stations communicate. Many types of communication networks have been developed. Conventionally, multiple stations are interconnected by cables. However, a wireless communication network that operates efficiently using radio frequency signals would be advantageous for many uses. In one type of wireless network, a master controller receives and transmits signals between all stations on the network. The master controller operates as an intermediary, arbitrating and allocating communication between the stations. However, a master controller introduces additional costs, and it would be an advantage if the wireless network did not utilize a master controller and thus would be "decentralized".
A decentralized, wireless communication system includes a plurality of stand-alone stations that communicate with radio frequency signals. If a practical system could be developed to operate efficiently, it would be particularly useful for an in-door communication system between stand-alone computer stations, for example in an office environment.
To communicate, simple conventional radio systems use a single dedicated radio frequency. However, the number of available radio frequencies is limited, and as the number of stations increase, the probability of a collision between single frequency signals becomes more and more likely. Other significant problems with single frequency systems include interference from other sources of RF energy and fading due to environmental factors, either of which can render all or a part of a message unreadable. For example, the indoor radio environment is characterized by potentially severe multipath fading, together with large propagation losses. Multipath fading causes the signal-to-noise ratio (SNR) vs. bit-error ratio (BER) curves to differ significantly from their flee-space characteristics. It has been experimentally determined that the indoor radio environment at about 1.0 GHz can be modeled as a very slowly varying, frequency-selective, Rayleigh fading environment. The movement of people, objects, etc. within the building causes the signal amplitude at a point to be slowly varying. Even with a dedicated radio frequency band, there are potentially serious problems with interference from other devices that emit radio frequency energy on the same channel during operation. Increasing transmission power is not a practical option because transmission power is limited by cost constraints and governmental regulation.
To provide a practical in-door communication system, it has been suggested to use a number (N) of radio frequency signals that are transmitted in a particular sequence known to the receiving stations. The technique of sequencing through N channels, when combined with coding techniques to be discussed, is called "cyclical slow frequency hopping and coding", and can be a useful technique to combat fading and interference in a wireless network. In a system using cyclical slow frequency hopping and coding, data to be transmitted is divided into segments, and each segment is transmitted on a different carrier frequency (channel). Burst error correction can be used when the data is re-assembled, thereby permitting the data to be correctly reconstituted even in the presence of fading or interference on one or more channels, even if up to e segments out of the N are corrupted. This technique is described in detail in "Indoor Radio Communications Using Time-Division Multiple Access with Cyclical Slow Frequency Hopping and Coding", by Adel A. M. Saleh and Leonard J. Cimini, Jr., IEEE Journal on Selected Areas in Communications, Vol. 7, No. 1, January 1989, pp. 59-70.
A RF Data Transport Protocol is required to send and receive data in a decentralized Local Area Network (LAN), in which all nodes are peers, using a slow frequency hopping communication system requires an RF Data Transport Protocol that will accurately move packets from the transmitter to the receiver under the control of a higher layer Media Access Control (MAC) protocol. Token passing protocols, in which one or more nodes are dynamically designated as a master node on the LAN, are not suitable, given the possibility of hidden nodes and the (semi) mobile nature of the nodes on the LAN. It has been suggested to use time division multiplexing to implement a system with cyclical slow frequency hopping. In a system with time division multiplexing, each station has a designated time slot in which it can transmit. However, time division multiplexing requires precise synchronization between all the machines on the network, a requirement that is difficult to meet in a decentralized system because the receiving stations do not know when to begin receiving the transmission, nor do they know when to start transmitting.
An appropriate MAC protocol for a decentralized LAN is described in the patent application cross-referenced above, entitled "Scanning Method for Receiving a Transmission in a Communication System with Slow Frequency Hopping and Coding" by Paulette R. Altmaier and Peter J. Potrebic, which describes a system in which receiving stations scan the first (N-e) frequencies, and if a transmission is detected, they receive as much of the message as possible. The entire message is recoverable if (N-e) out of the N packets are recoverable. However, that system works only if (N-e) complete messages (packets) are received. If all of the bursts are degraded or corrupted to some extent, then the message will not be recoverable. It would be an advantage to provide a communication coding system that allows message recovery even if more than (N-e) of the packets are corrupted in some way. Such a system would be particularly useful in a wireless communication system.